Method for bonding a tantalum structure to a cobalt-alloy substrate

ABSTRACT

A method for bonding a porous tantalum structure to a substrate is provided. The method comprises providing a substrate comprising cobalt or a cobalt-chromium alloy; an interlayer consisting essentially of at least one of hafnium, manganese, niobium, palladium, zirconium, titanium, or alloys or combinations thereof; and a porous tantalum structure. Heat and pressure are applied to the substrate, the interlayer, and the porous tantalum structure to achieve solid-state diffusion between the substrate and the interlayer and between the interlayer and the porous tantalum structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional patent application of U.S.patent application Ser. No. 11/870,205, filed Oct. 10, 2007, which ishereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to orthopedic implants, and moreparticularly relates to a method for bonding a porous tantalum structureto cobalt or a cobalt-alloy orthopedic implant.

BACKGROUND OF THE INVENTION

Orthopedic implants are often utilized to help their recipients recoverfrom injury or disease. To promote quick recovery, orthopedic implantsare designed to cooperate with the body's natural inclination to healitself. Some orthopedic implants are designed to fosterosseointegration. As is known in the art, osseointegration is theintegration of living bone within a man-made material, usually a porousstructure. Cells in the recipient form new bone within the pores of theporous structure. Thus, the porous structure and the bone tissue becomeintermingled as the bone grows into the pores. Accordingly, orthopedicimplants may include a porous surface to enhance fixation between theorthopedic implant and adjacent tissue. Of course, the faster thesurrounding tissue grows into the porous surface, the sooner the patientmay begin to resume normal activities. However, the manufacture of theorthopedic implants with porous structures is not without difficulty,

Orthopedic implants are usually made from various metals. One difficultyencountered during manufacturing is bonding separate components, eachmade of a different metal, together. For example, cobalt is a popularmetal used to make orthopedic implants, and other popular metals includealloys of cobalt with other metals, such as chromium. The porousstructure may be made from an entirely different metal, such astantalum. In this case, bonding the porous metal to the orthopedicimplant involves bonding tantalum to cobalt or to cobalt-chromiumalloys. Bonding these two metals together has proved to be particularlyproblematic.

Thus, there is a need for an improved method of bonding of porousstructures, specifically tantalum, to cobalt and cobalt- alloy implantssuch that the bond has sufficient strength while the corrosionresistance of the metals in the resulting implant are maintained.

SUMMARY OF THE INVENTION

The present invention provides a method for bonding a porous tantalumstructure to a substrate. In one embodiment, the method comprisesproviding (i) a substrate comprising cobalt or a cobalt-chromium alloy;(ii) an interlayer consisting essentially of at least one of hafnium,manganese, niobium, palladium, zirconium, titanium, or alloys orcombinations thereof; and (iii) a porous tantalum structure, andapplying heat and pressure for a time sufficient to achieve solid-statediffusion between the substrate and the interlayer and solid-statediffusion between the interlayer and the porous tantalum structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description given below, serve to explain the invention.

FIG. 1 depicts a cross-sectional view of one embodiment of an assemblycomprising a porous tantalum structure, a pre-formed sheet interlayer,and a substrate;

FIG. 2 depicts a cross-sectional view of another embodiment of anassembly comprising a porous tantalum structure, a coating interlayer,and a substrate; and

FIGS. 3 and 4 are photomicrographs corresponding to the embodiments ofFIGS. 1 and 2, respectively, following heating and pressing the assemblyto bond the porous tantalum structure to the interlayer and theinterlayer to the substrate.

DETAILED DESCRIPTION

In accordance with the present invention and with reference to FIGS. 1and 2, a method for bonding a porous tantalum structure 10 to asubstrate 12 generally begins by constructing an assembly 14 comprisingan interlayer 16 placed on the surface of the substrate 12 and theporous tantalum structure 10 placed onto the interlayer 16. It will beappreciated that the assembly 14 may be constructed by placing theindividual components 10, 12, 16 together in any order that results inthe interlayer 16 positioned between and in contact with the substrate12, and the porous tantalum structure 10, as shown in FIGS. 1 and 2. Inother words, the placement order is not limited to those ordersdescribed herein.

The porous tantalum structure 10 may be TRABECULAR METAL®, availablefrom Zimmer Inc., Warsaw, Ind. The porous tantalum structure 10 isconfigured to facilitate osseointegration. The porous tantalum structure10 may have a pore size, pore continuity, and other features forfacilitating bone tissue growth into the pores, as is known in the art.

The substrate 12 may be a cast or a wrought cobalt or cobalt chromiumalloy fabricated in a shape according to the requirements for thespecific orthopedic application. For example, the substrate 12 may becast of cobalt in the shape of a total hip replacement implant. Otherimplants may include implants for the ankle, elbow, shoulder, knee,wrist, finger, and toe joints or other portions of the body that maybenefit from a substrate 12 having a porous tantalum structure 10 bondedthereto.

With no intent to be bound by theory, tantalum and cobalt metals are notreadily soluble, that is, the documented solid solubility of tantaluminto cobalt is insufficient to form the necessary bond strength demandedby applications within the human body. In fact, certain stoichiometriesof tantalum with cobalt may prevent solid-state diffusion of tantaluminto cobalt and vice versa. Therefore, in accordance with the method ofthe present disclosure, the interlayer 16 comprises a metal that readilyforms solid solutions with both tantalum and cobalt or cobalt-chromiumalloys. For example, the interlayer 16 may be any one or an alloy ofmetals, such as, hafnium, manganese, niobium, palladium, zirconium,titanium, or other metals or alloys that exhibit solid solubility withtantalum at temperatures less than the melting temperature of thesubstrate 12, the interlayer 16, or the porous tantalum structure 10.

The assembly 14, as shown in FIGS. 1 and 2, may be put together byapplying the interlayer 16 to the substrate 12. One skilled in the artwill observe that the interlayer 16 may require pre-shaping to improvethe contact area between the surface of the substrate 12 and the surfaceof interlayer 16 prior to applying the interlayer 16 to the substrate12. Alternatively, the interlayer 16 may be press formed onto thesubstrate 12 such that the interlayer 16 conforms to the surface of thesubstrate 12. The surfaces of all components 10, 12, 16 may be cleanedprior to assembly 14 to reduce corrosion and improve solid-statediffusion bonding.

With continued reference to FIGS. 1 and 2, following application of theinterlayer 16 to the substrate 12, the porous tantalum structure 10 maybe placed on the interlayer 16 thus forming the assembly 14. Similar topre-shaping the interlayer 16 to conform to the substrate 12, the poroustantalum structure 10 may be formed in a shape to maximizesurface-to-surface contact to facilitate solid-state diffusion with theinterlayer 16.

Heat and pressure are applied to the assembly 14 sufficient forsolid-state diffusion to take place between the substrate 12 and theinterlayer 16 and between the interlayer 16 and the porous tantalumstructure 10. As is known to those skilled in the art, solid-statediffusion is the movement and transport of atoms in solid phases.Solid-state diffusion bonding forms a monolithic joint through formationof bonds at an atomic level due to transport of atoms between two ormore metal surfaces. Heat and pressure may be supplied to the assembly14 with a variety of methods known in the art. For example, the assembly14 may be heated electrically, radiantly, optically, by induction, bycombustion, by microwave, or other means known in the art. Pressure maybe applied mechanically by clamping the assembly 14 together prior toinsertion of the assembly 14 into a furnace, or pressure may be appliedvia a hot pressing system capable of applying pressure once the assembly14 reaches a target temperature, as is known in the art. Furthermore,hot pressing may include hot isostatic pressing, also known in the art.

Referring now to FIG. 1, in one embodiment, the interlayer 16 is apre-formed sheet of commercially pure titanium at least about 0.016inches (about 0.04064 centimeter) thick. In another embodiment, thepre-formed sheet of commercially pure titanium is at least about 0.020inches (about 0.0508 centimeter) thick for improved bond strength. Itwill be observed that the interlayer 16 may be positioned directlybeneath the porous tantalum structure 10. In other words, it is notnecessary to cover the entire substrate 12 with the interlayer 16 tobond the porous tantalum structure 10 at a single location. Furthermore,it will also be observed that the corrosion resistance and the strengthof the substrate 12 are not negatively impacted if the porous tantalumstructure 10 touches those areas not covered by the interlayer 16 duringheating. Thus, the porous tantalum structure 10 may be bonded tomultiple separate areas on the surface of the substrate 12 with multipleseparate areas of interlayer 16. One skilled in the art will appreciatethat the position of the porous tantalum structure 10 may be dictated bythe patient's physiological requirements.

In one embodiment, the assembly 14 is clamped together by applying apressure of at least approximately 200 pounds per square inch (psi)(approximately 1.38 MPa). However, pressures greater than approximately200 psi may be applied up to the compressive yield strength of the anyof the substrate 12, the interlayer 16, or the porous tantalum structure10. Ordinarily, the porous tantalum structure 10 has the lowestcompressive yield strength, for example, 5,800 psi for TRABECULARMETAL®.

The clamped assembly 14 is then heated to at least about 540° C. (about1004 degree Fahrenheit) in vacuum or in another sub-atmospheric pressureof an inert atmosphere. In any case, the clamped assembly 14 is heatedto less than the melting temperature of any of the components 10, 12, 16and, in most cases, is at least about 800° C. (about 1472 degreeFahrenheit) but less than about 1000° C. (about 1832 degree Fahrenheit)in vacuum. One skilled in the art will observe that the higher thetemperature, the less time it will take to achieve solid-state diffusionbonding. The time required to achieve solid-state diffusion bonding maybe as little as less than 1 hour to as long as 48 hours and will dependon the metals involved, the temperatures, atmosphere, and the pressuresapplied.

Once heated to temperature, and after a time sufficient to achievesolid-state diffusion between the porous tantalum structure 10 and theinterlayer 16 and between the interlayer 16 and the substrate 12, aconstruct is formed. The construct may comprise the substrate 12 bondedto the interlayer 16 and the interlayer 16 bonded to the porous tantalumstructure 10. FIG. 3 is a photomicrograph of a portion of the constructformed according to one embodiment of the method, described above, witha porous tantalum structure 10 (top) bonded to a titanium sheetinterlayer 16 (middle) bonded to a cobalt-chromium substrate 12(bottom).

With reference now to FIG. 2, in another embodiment, the interlayer 16is a coating applied to the surface by, for example, thermal spray,plasma spray, electron beam deposition, laser deposition, cold spray, orother method of forming the coatings on a substrate 12. In one exemplaryembodiment, the coating interlayer 16 is applied via vacuum plasmaspraying, as is known in the art. The substrate 12 may be masked andthen grit blasted to prepare the surface of the substrate 12 for vacuumplasma spraying. In one exemplary embodiment, the substrate 12 is maskedand then grit blasted with alumina (aluminum oxide) grit for increasedcorrosion resistance of the construct subsequent to bonding with theinterlayer 16. In another exemplary embodiment, the coating interlayer16 comprises titanium sprayed to a thickness of at least about 0.010inches (about 0.0254 centimeter) thick. In another embodiment, forincreased bond strength, the titanium coating interlayer 16 is at leastabout 0.020 inches (about 0.0508 centimeter) thick. In the vacuum plasmasprayed embodiments, a porosity level is between about 20% and about 40%for ease of vacuum plasma spray processing while maintaining sufficientcorrosion resistance. FIG. 4 is a photomicrograph of a portion of aconstruct formed according to one embodiment of the method describedabove, showing a portion of a construct comprising a porous tantalumstructure 10 (top) bonded to a titanium vacuum plasma sprayed interlayer16 (middle) bonded to a cobalt-chromium substrate 12 (bottom).

In one exemplary embodiment, a construct comprising a porous tantalumstructure 10 of TRABECULAR METAL® bonded to a titanium interlayer 16bonded to a cobalt-chromium substrate 12 was characterized by tensilestrength testing. Nearly all failure separations occurred in the poroustantalum structure 10. Tensile stresses measured at separation onconstructs formed according to the previously described embodiments wereroutinely above 2,900 psi.

One skilled in the art will observe that heating and applying pressuremay include multiple heating and pressurizing processes. For example,the porous tantalum structure 10 may be assembled with the interlayer 16and bonded thereto, according to one embodiment of the method, to form asubassembly. That subassembly may then be bonded to the substrate 12according to another embodiment of the method. The reverse procedure mayalso be used. That is, the interlayer 16 may be bonded to the substrate12 to form a subassembly with subsequent bonding of the porous tantalumstructure 10 to the interlayer portion of the subassembly. Therefore,embodiments of the method may account for different diffusioncoefficients between the components 10, 12, 16 which may allow for moreconsistent, higher strength bonds between the substrate 12 andinterlayer 16 and between the interlayer 16 and the porous tantalumstructure 10. By way of further example and not limitation, diffusionbonding of a titanium interlayer 16 to a cobalt-chromium substrate 12 atan elevated temperature and pressure may take longer than diffusionbonding of the titanium interlayer 16 to a porous tantalum structure 10at similar pressures and temperatures. Thus, by diffusion bonding thetitanium interlayer 16 to the cobalt-chromium substrate 12 to form asubassembly and then diffusion bonding the porous tantalum structure 10to the subassembly, a diffusion bond depth between the titaniuminterlayer 16 and the cobalt-chromium substrate 12 may be substantiallythe same as a diffusion bond depth between the titanium interlayer 16and the porous tantalum structure 10. In contrast, if the poroustantalum structure 10, the titanium interlayer 16, and thecobalt-chromium substrate 12 are bonded with a single application ofheat and pressure, the diffusion bond depths between the titaniuminterlayer 16 and the porous tantalum structure 10 and between thetitanium interlayer 16 and the cobalt-chromium substrate 12 may bedifferent.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus and methodand illustrative examples shown and described. Accordingly, departuresmay be made from such details without departing from the scope of thegeneral inventive concept.

1.-25. (canceled)
 26. A method for bonding a porous tantalum structureto a substrate, comprising: providing a substrate comprising cobalt or acobalt-chromium alloy; providing a porous tantalum structure; applyingan interlayer consisting essentially of at least one of hafnium,manganese, niobium, palladium, zirconium, titanium, or alloys orcombinations thereof to a surface of one of the porous tantalumstructure and the substrate, wherein the interlayer is applied to thesurface by at least one of thermal spraying, plasma spraying, electronbeam deposition, laser deposition, chemical vapor deposition,electrodeposition, or cold spray coating; positioning the other of theporous tantalum structure and the substrate in contact with theinterlayer thereby forming an assembly, wherein with the interlayer isbetween the substrate and the porous tantalum structure; and applyingheat and pressure to the assembly for a time sufficient to achievesolid-state diffusion between the substrate and the interlayer andsolid-state diffusion between the interlayer and the porous tantalumstructure.
 27. The method of claim 26 wherein the interlayer is at leastabout 0.016 inches thick.
 28. The method of claim 26 wherein theinterlayer is at least about 0.010 inches thick.
 29. The method of claim26 wherein the interlayer has a porosity of between about 20% and about40%.
 30. The method of claim 26 wherein applying the interlayer includesplasma spraying the interlayer onto the surface in at least a partialvacuum.
 31. The method of claim 26 wherein applying pressure includesapplying at least approximately 200 psi to the assembly.
 32. The methodof claim 26 wherein applying pressure includes applying a pressure thatis less than a compressive yield strength of the porous tantalumstructure.
 33. The method of claim 26 wherein heating includes heatingto less than about 1000° C. in a vacuum environment.
 34. The method ofclaim 26 wherein the interlayer has a thickness of at least about 0.010inches and applying heat and pressure includes applying a pressure ofleast about 200 psi and heating the assembly to at least about 540° C.for at least one hour to achieve solid-state diffusion between thesubstrate and the interlayer and between the interlayer and the poroustantalum structure.
 35. A method for bonding a porous tantalum structureto a substrate, comprising: providing a substrate comprising cobalt or acobalt-chromium alloy; applying an interlayer consisting essentially ofat least one of hafnium, manganese, niobium, palladium, zirconium,titanium, or alloys or combinations thereof to a surface of thesubstrate, wherein the interlayer is applied to the surface by at leastone of thermal spraying, plasma spraying, electron beam deposition,laser deposition, chemical vapor deposition, electrodeposition, or coldspray coating; applying heat and pressure to the interlayer and thesubstrate for a time sufficient to achieve solid-state diffusion betweenthe substrate and the interlayer thereby forming a subassembly;positioning a porous tantalum structure in contact with the interlayerportion of the subassembly thereby forming an assembly; and applyingheat and pressure to the assembly for a time sufficient to achievesolid-state diffusion between the interlayer and the porous tantalumstructure.
 36. The method of claim 35 wherein the interlayer is at leastabout 0.010 inches thick.
 37. The method of claim 35 wherein applyingthe interlayer includes plasma spraying the interlayer onto the surfaceof the substrate in at least a partial vacuum.
 38. The method of claim35 wherein the interlayer has a porosity of between about 20% and about40%.
 39. A method for bonding a porous tantalum structure to asubstrate, comprising: providing a porous tantalum structure; applyingan interlayer consisting essentially of at least one of hafnium,manganese, niobium, palladium, zirconium, titanium, or alloys orcombinations thereof to a surface of the porous tantalum structurewherein the interlayer is applied to the surface by at least one ofthermal spraying, plasma spraying, electron beam deposition, laserdeposition, chemical vapor deposition, electrodeposition, or cold spraycoating; applying heat and pressure to the interlayer and the poroustantalum structure for a time sufficient to achieve solid-statediffusion between the porous tantalum structure and the interlayerthereby forming a subassembly; positioning a substrate comprising cobaltor a cobalt-chromium alloy in contact with the interlayer portion of thesubassembly thereby forming an assembly; and applying heat and pressureto the assembly for a time sufficient to achieve solid-state diffusionbetween the interlayer and the substrate.
 40. The method of claim 39wherein the interlayer is at least about 0.010 inches thick.
 41. Themethod of claim 39 wherein applying the interlayer includes plasmaspraying the interlayer onto the surface of the substrate in at least apartial vacuum.
 42. The method of claim 10 wherein the interlayer has aporosity of between about 20% and about 40%.
 43. The method of claim 39wherein applying pressure to the assembly includes applying at leastapproximately 200 psi to the assembly.
 44. The method of claim 39wherein applying pressure to the assembly includes applying a pressurethat is less than a compressive yield strength of the porous tantalumstructure.
 45. The method of claim 39 wherein applying heat to theassembly includes heating to less than about 1000° C. in a vacuumenvironment.